Polyol-based method for forming thin film aerogels on semiconductor substrates

ABSTRACT

This invention has enabled a new, simple nanoporous dielectric fabrication method. In general, this invention uses a polyol, such as glycerol, as a solvent. This new method allows both bulk and thin film aerogels to be made without supercritical drying, freeze drying, or a surface modification step before drying. Prior art aerogels have required at least one of these steps to prevent substantial pore collapse during drying. Thus, this invention allows production of nanoporous dielectrics at room temperature and atmospheric pressure, without a separate surface modification step. Although not required to prevent substantial densification, this new method does not exclude the use of supercritical drying or surface modification steps prior to drying. In general, this new method is compatible with most prior art aerogel techniques. Although this new method allows fabrication of aerogels without substantial pore collapse during drying, there may be some permanent shrinkage during aging and/or drying.

FIELD OF THE INVENTION

This invention pertains generally to precursors and deposition methodsfor thin films aerogels on semiconductor substrates, includingdeposition methods suited to aerogel thin film fabrication of nanoporousdielectrics.

BACKGROUND OF THE INVENTION

Semiconductors are widely used in integrated circuits for electronicdevices such as computers and televisions. These integrated circuitstypically combine many transistors on a single crystal silicon chip toperform complex functions and store data. Semiconductor and electronicsmanufacturers, as well as end users, desire integrated circuits whichcan accomplish more in less time in a smaller package while consumingless power. However, many of these desires are in opposition to eachother. For instance, simply shrinking the feature size on a givencircuit from 0.5 microns to 0.25 microns can increase power consumptionby 30%. Likewise, doubling operational speed generally doubles powerconsumption. Miniaturization also generally results in increasedcapacitive coupling, or crosstalk, between conductors which carrysignals across the chip. This effect both limits achievable speed anddegrades the noise margin used to insure proper device operation.

One way to diminish power consumption and crosstalk effects is todecrease the dielectric constant of the insulator, or dielectric, whichseparates conductors. Probably the most common semiconductor dielectricis silicon dioxide, which has a dielectric constant of about 3.9. Incontrast, air (including partial vacuum) has a dielectric constant ofjust over 1.0. Consequently, many capacitance-reducing schemes have beendevised to at least partially replace solid dielectrics with air.

U.S. Pat. No. 4,987,101, issued to Kaanta et al., on Jan. 22, 1991,describes a method for fabricating gas (air) dielectrics, whichcomprises depositing a temporary layer of removable material betweensupports (such as conductors), covering this with a capping insulatorlayer, opening access holes in the cap, extracting the removablematerial through these access holes, then closing the access holes.

U.S. Pat. No. 5,103,288, issued to Sakamoto, on Apr. 7, 1992, describesa multilayered wiring structure which decreases capacitance by employinga porous dielectric. This structure is typically formed by depositing amixture of an acidic oxide and a basic oxide to form a non-porous solid,heat treating to precipitate the basic oxide, and then dissolving outthe basic oxide to form a porous solid. Dissolving all of the basicoxide out of such a structure may be problematic, because small pocketsof the basic oxide may not be reached by the leaching agent.Furthermore, several of the elements described for use in thisnon-gel-based method (including sodium and lithium) are generallyconsidered contaminants in the semiconductor industry, and as such areusually avoided in a production environment. Creating only extremelysmall pores (less than 10 nm) may be difficult using this method, yetthis requirement will exist as submicron processes continue to scaletowards a tenth of a micron and less.

Another method of forming porous dielectric films on semiconductorsubstrates (the term "substrate" is used loosely herein to include anylayers formed prior to the conductor/insulator level of interest) isdescribed in U.S. Pat. No. 4,652,467, issued to Brinker et al., on Mar.24, 1987. This patent teaches a sol-gel technique for depositing porousfilms with controlled porosity and pore size (diameter), wherein asolution is deposited on a substrate, gelled, and then crosslinked anddensified by removing the solvent through evaporation, thereby leaving adry, porous dielectric. This method has as a primary objective thedensification of the film, which teaches away from low dielectricconstant applications. Dielectrics formed by this method are typically15% to 50% porous, with a permanent film thickness reduction of at least20% during drying. The higher porosities (e.g. 40%-50%) can only beachieved at pore sizes which are generally too large for suchmicrocircuit applications. These materials are usually referred to asxerogels, although the final structure is not a gel, but an open-pored(the pores are generally interconnected, rather than being isolatedcells) porous structure of a solid material.

As shown in the Brinker patent, semiconductor fabricators have usedsol-gel techniques to produce dense thin films in semiconductors. Theword sol-gel, however, does not describe a product but a reactionmechanism whereby a sol transforms into a gel. A sol is a colloidalsuspension of solid particles in a liquid. One method of forming a solis through hydrolysis and condensation reactions. These reactions causea multifunctional monomer in a solution to polymerize into relativelylarge, highly branched particles. Many monomers suitable forpolymerization are metal alkoxides. For example, atetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed inwater by the reaction

    Si(OEt).sub.4 +H.sub.2 O→HO--Si(OEt).sub.3 +EtOH

Reaction conditions may be controlled such that, on the average, eachmonomer undergoes a desired number of hydrolysis reactions to partiallyor fully hydrolyze the monomer. TEOS which has been fully hydrolyzedbecomes Si(OH)₄. Once a molecule has been at least partially hydrolyzed,two molecules can then link together in a condensation reaction, such as

    (OEt).sub.3 Si--OH+HO--Si(OH).sub.3 →(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O

or

    (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3 →(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH

to form an oligomer and liberate a molecule of water or ethanol. TheSi--O--Si configuration in the oligomer formed by these reactions hasthree sites available at each end for further hydrolysis andcondensation. Thus, additional monomers or oligomers can be added tothis molecule in a somewhat random fashion to create a highly branchedpolymeric molecule from literally thousands of monomers.

One theory is, that through continued reactions, one or more moleculesin the sol may eventually reach macroscopic dimensions so that it/theyform a network which extends substantially throughout the sol. At thispoint (called the gel point), the substance is said to be a gel. By thisdefinition, a gel is a substance that contains a continuous solidskeleton enclosing a continuous liquid phase. As the skeleton is porous,a gel can also be described as an open-pored solid structure enclosing apore fluid. An oligomerized metal alkoxide, as defined herein, comprisesmolecules formed from at least two alkoxide monomers, but does notcomprise a gel.

In a typical thin film xerogel process, an ungelled precursor sol may beapplied to (e.g., spray coated, dip-coated, or spin-coated) a substrateto form a thin film on the order of several microns or less inthickness, gelled, and dried to form a dense film. The precursor soloften comprises a stock solution, a solvent, and a gelation catalyst.This catalyst typically modifies the pH of the precursor sol in order tospeed gelation. In practice, such a thin film is subjected to rapidevaporation of volatile components. Thus, the deposition, gelation, anddrying phases may take place simultaneously (at least to some degree) asthe film collapses rapidly to a dense film. Drying by evaporation of thepore fluid produces extreme capillary pressure in the microscopic poresof the wet gel. This pressure typically causes many pores to collapseand reduces the gel volume as it dries, typically by an order ofmagnitude or more.

A dried gel that is formed by collapsing and densifying a wet gel duringdrying has been termed a xerogel. Typical thin film xerogel methodsproduce gels having limited porosity (Up to 60% with large pore sizes,but generally substantially less than 50% with pore sizes of interest).An aerogel is distinguishable from a xerogel primarily by largelyavoiding pore collapse during drying of the wet gel.

U.S. Pat. No. 5,470,802, A Low Dielectric Constant Material ForElectronics Applications, issued on Nov. 28, 1995 to Gnade, Cho andSmith describes a method for forming highly porous, finely pored (porediameter of less than 80 nm and preferably of 2 nm to 25 nm), lowdielectric constant (k less than 3.0 and preferably less than 2.0)dielectric films for use as semiconductor insulators. The U.S. '802invention uses a surface modification agent to control densification andother shrinkage effects during drying, resulting in a substantiallyundensified, highly porous rigid structure which can be processed atatmospheric pressure. U.S. '802 teaches that the porous structure can bemade hydrophobic (water repelling) and that the pores formed in thedielectric can be made small enough to allow this method to be used withdevice feature sizes in the 0.5 to 0.1 micron range, or even smaller.This results in a thin film that can be fabricated with almost anydesired porosity (thin films with greater than 90% porosity have beendemonstrated). Such films have been found to be desirable for a lowdielectric constant insulation layer in microelectronic applications.

These techniques relate to fabricating dielectric (electricallynonconductive) materials, usually inorganic dielectrics. The inorganicporous dielectrics "aerogels" are nanoporous having average pore sizesless than 250 nanometers (preferably less than 50 nanometers and morepreferably less than 10 nanometers and still more preferably less than 5nanometers). Nanoporous dielectrics are of particular interest inadvanced semiconductor manufacturing. The nanoporous inorganicdielectrics include the nanoporous metal oxides, particularly nanoporoussilica.

Gnade et al.'s teachings include a subcritical drying method. That is,they dry the gelled film at one or more sub-critical pressures (fromvacuum to near-critical) and preferably, at atmospheric pressure.Traditional aerogel processes typically replace the pore fluid with adrying fluid such as ethanol or CO₂. The traditional processes thenremove the drying fluid from a wet gel (dry) under supercriticalpressure and temperature conditions. By removing the fluid in thesupercritical region, vaporization of liquid does not take place.Instead, the fluid undergoes a constant change in density during theoperation, changing from a compressed liquid to a superheated vapor withno distinguishable state boundary. This technique avoids the capillarypressure problem entirely, since no state change boundaries ever existin the pores.

SUMMARY OF THE INVENTION

Copending U.S. patent application Ser. No. 08/746,679, titled AerogelThin Film Formation From Multi-Solvent Systems, by Smith et al. teachesa method of varying the precursor sol viscosity independently of thedried gel density. This multi-solvent method comprises the step ofdepositing a thin film of an aerogel precursor sol on a semiconductorsubstrate; the sol comprises a reactant, which may be a partiallypolymerized metal alkoxide or other precursor, dispersed in a firstsolvent and a second solvent. The method further comprisespreferentially evaporating substantially all of the second solvent fromthe thin film, preferably without substantial evaporation of the firstsolvent, and subsequently allowing the thin film to cross-link, thusforming a wet gel having pores arranged in an open-pored structure onthe semiconductor substrate. This multi-solvent method allows theprecursor sol viscosity to be varied independently of the dried geldensity. However, it still generally requires some method, such asatmospheric control, to limit evaporation of the first solvent.

In principle, this evaporation rate control can be accomplished bycontrolling the solvent vapor concentration above the wafer. However,our experience has shown that the solvent evaporation rate is verysensitive to small changes in the vapor concentration and temperature.In an effort to better understand this process, we have modeledisothermal solvent vaporization from a wafer as a function of percentsaturation. This modeling is based on basic mass transfer theory.Transport Phenomena, (particularly Chapters 16 and 17) by R. B. Bird, W.E. Stewart, and E. N. Lightfoot, is a good reference for mass transfertheory. These calculations were performed for a range of solvents. Theambient temperature evaporation rates for some of these solvents aregiven in FIG. 1. For evaporation to not be a processing problem, theproduct of the evaporation rate and processing time (preferably on theorder of minutes) must be significantly less than the film thickness.This suggests that for solvents such as ethanol, the atmosphere abovethe wafer would have to be maintained at over about 99% saturation.However, there can be problems associated with allowing the atmosphereto reach saturation or supersaturation. Some of these problems arerelated to condensation of an atmospheric constituent upon the thinfilm. Condensation on either the gelled or ungelled thin film has beenfound to cause defects in an insufficiently aged film. Thus, it isgenerally desirable to control the atmosphere such that no constituentis saturated.

Rather than using a high volatility solvent and precisely controllingthe solvent atmosphere, it has been discovered that a better solution isto use a low volatility solvent with less atmospheric control. Tosimplify atmospheric control, it may be desirable to have at least athree degrees C. (or more preferably, 10 degrees C.) difference betweenthe condensation temperature of the solvent vapor and the substrate.Viscosity during deposition can be controlled either by heating/coolingthe precursor sol or by combining this new approach with themulti-solvent approach described above. Although it is preferable toanalyze a solvent to determine its expected evaporation rate, apreliminary preference on the selection of the low volatility solventcan be made. Preferably the low volatility solvent is one with a boilingpoint in the 175°-300° C. range and (for TEOS based gels) that it bemiscible with both water and ethanol. Thus, some suitable low volatilitysolvent candidates are polyols, these preferred polyols includetrihydric alcohols, such as glycerol and glycols (dihydric alcohols),such as ethylene glycol, 1,4-butylene glycol, and 1,5-pentanediol. Ofthese, the most economical are ethylene glycol and glycerol.

The use of a polyol allows a loosening (as compared to prior artsolvents) of the required atmospheric control during deposition and/orgelation. This is because, that even though saturation should stillpreferably be avoided, the atmospheric solvent concentration can belowered without excessive evaporation. FIG. 5 shows how the evaporationrate of ethylene glycol varies with temperature and atmospheric solventconcentration. FIG. 11 shows how the evaporation rate of glycerol varieswith temperature and atmospheric solvent concentration. It has been ourexperience that, with polyols, acceptable gels can be formed bydepositing and gelling in an uncontrolled or a substantiallyuncontrolled atmosphere. In this most preferred approach (asubstantially uncontrolled atmosphere) atmospheric controls, if any,during deposition and gelation are typically limited to standardcleanroom temperature and humidity controls, although the wafer and/orprecursor sol may have independent temperature controls.

One attractive feature of using a polyol as a solvent is that at ambienttemperature, the evaporation rate is sufficiently low so that severalminutes at ambient conditions will not yield dramatic shrinkage for thinfilms. However, in addition to serving as a low vapor pressure andwater-miscible solvent, polyols may also participate in sol-gelreactions. Although the exact reactions in this process have not beenfully studied, some reactions can be predicted. If tetraethoxysilane(TEOS) is employed as a precursor with an ethylene glycol solvent, theethylene glycol can exchange with the ethoxy groups:

    Si(OC.sub.2 H.sub.5).sub.4 +x HOC.sub.2 H.sub.4 OH←→Si(OC.sub.2 H.sub.5).sub.4-x (OC.sub.2 H.sub.4 OH).sub.x +x C.sub.2 H.sub.5 OH

Similarly, if tetraethoxysilane (TEOS) is employed as a precursor with aglycerol solvent, the glycerol can exchange with the ethoxy groups:

    Si(OC.sub.2 H.sub.5).sub.4 +x HOCH.sub.2 CH(OH)CH.sub.2 OH!←→Si(OC.sub.2 H.sub.5).sub.4-x  OC.sub.3 H.sub.5 (OH).sub.2 !.sub.x +x C.sub.2 H.sub.5 OH!

In principle, the presence and concentration of these chemical groupscan change the precursor reactivity (i.e., gel time), modify the gelmicrostructure (surface area, pore size distribution, etc.), change theaging characteristics, or change nearly any other characteristic of thegel.

Ethylene glycol and glycerol could react with TEOS and produce a driedgel with surprisingly different properties than that of an ethanol/TEOSgel. Unanticipated property changes in the ethylene glycol/TEOS basedgels and the glycerol/TEOS based gels generally include (at least onmost formulations):

Lower density is achievable without supercritical drying or pre-dryingsurface modification

Shorter gel times at a given catalyst content

Strengths of bulk samples which are approximately an order of magnitudegreater (at a given density) than conventional TEOS gels

Very high surface area (˜1,000 m² /g)

High optical clarity of bulk samples (This is likely due to a narrowerpore size distribution than conventional TEOS gels)

Low density--With this invention, it is possible to form dried gels atvery low densities without pre-drying surface modification orsupercritical drying. These low densities can generally be down around0.3 to 0.2 g/cm³ (non-porous SiO₂ has a density of 2.2 g/cm³), or withcare, around 0.1 g/cm³. Stated in terms of porosity (porosity is thepercentage of a structure which is hollow), this denotes porosities ofabout 86% and 91% (about 95% porosity with a density of 0.1 g/cm³). Asshown in FIG. 7, these porosities correspond to dielectric constants ofabout 1.4 for the 86% porous, and 1.2 for 91% porous. The actualmechanism that allows these high porosities is not fully known. However,it may be because the gels have high mechanical strength, because thegels do not have as many surface OH (hydroxyl) groups, a combination ofthese, or some other factors. This method also obtains excellentuniformity across the wafer. FIG. 6 shows the refractive index (and thusthe porosity) at several locations on a sample semiconductor substrate.

If desired, this process can be adjusted (by varying the TEOS/solventratios) to give any porosity from above 90% down to about 20, or even10%. Typical prior art dried gels with small pore sizes required eithersupercritical drying or a surface modification step before drying toachieve these low densities. While some prior art xerogels haveporosities greater than 50%; these prior art xerogels had substantiallylarger pore sizes (typically above 100 nm). These large pore size gelshave less mechanical strength. Additionally, their large size makes themunsuitable for filling small (typically less than 1 μm) patterned gapson a microcircuit.

Thus, this invention has enabled a new, simple nanoporous low densitydielectric fabrication method. This new polyol-based method allows bothbulk and thin film aerogels to be made without supercritical drying, ora surface modification step before drying. Prior art aerogels haverequired at least one of these steps to prevent substantial porecollapse during drying.

Density Prediction--By varying the ratio of ethylene glycol (EG) toethanol (EtOH) in the precursor (at a fixed silica content), the densityafter ethanol/water evaporation can be calculated. This is likely due tothe well controlled evaporation allowed by the low volatility solvent.To the extent that further shrinkage is prevented during aging anddrying, this allows prediction of the density (and thus porosity) of thedried gel. Although this density prediction had generally not been alarge problem with bulk gels, thin film gels had typically neededexcellent atmospheric controls to enable consistent density predictions.Table 1 shows the predicted and actual density for three differentEG/EtOH ratios after substantial ethanol and water removal, but beforedrying (EG removal).

                  TABLE 1                                                         ______________________________________                                        Correlation between predicted and                                             measured density of wet bulk gels                                             after ethanol/water evaporation.                                                              Predicted                                                                     Density  Density (g/cm.sup.3)                                 Stock Solution  (g/cm.sup.3)                                                                           after 80° C.                                  ______________________________________                                        40%EtOH/60%EG   0.37     0.40                                                 51%EtOH/49%EG   0.43     0.45                                                 60%EtOH/40%EG   0.53     0.50                                                 ______________________________________                                    

To some degree, the glycerol-based processes behave similarly to theethylene glycol-based processes. However, the ethylene glycol-based gelsoften have significant evaporation during aging. The glycerol-based gelshave dramatically lower evaporation and shrinkage rates during aging.This allows atmospheric control to be loosened during aging. We havefabricated acceptable glycerol-based gels with no atmospheric controlsduring aging.

Shorter Gel Times--In addition to enabling prediction of the density,the use of polyols may also change other properties of the sol-gelprocess. FIG. 2 shows gel times for two different ethylene glycol-basedcompositions as a function of the amount of ammonia catalyst used. Thesegel times are for bulk gels for which there is no evaporation of ethanoland/or water as there would be for thin films. Evaporation increases thesilica content and thus, decreases the gel time. Therefore, these gelstimes may be the upper limit for a given precursor/catalyst. The geltimes reported in FIG. 2 are approximately an order of magnitude shorterthan precursors without a polyol. Gel times are generally also a firstorder dependence on the concentration of ammonia catalyst. This impliesthat it may be possible to easily control the gel times. For thin filmsof these new polyol-based gels, it is routine to obtain gelation withinminutes, even without a gelation catalyst.

Higher Strength--The properties of the polyol-based samples appear to bequite different from regular gels as evidenced by both their low degreeof drying shrinkage and differences in qualitative handling of the wetand dry gels. Thus, upon physical inspection, both the glycerol-basedand ethylene glycol-based dried gels seem to have improved mechanicalproperties as compared to conventional dried gels. We have compared thebulk modulus measured during isostatic compaction measurements of onesample prepared using one ethylene glycol-based and one conventionalethanol-based dried bulk gel (both have the same initial density). Afterinitial changes attributed to buckling of the structure, both samplesexhibit power law dependence of modulus with density. This power lawdependence is usually observed in dried gels. However, what issurprising is the strength of the ethylene glycol-based dried gel. At agiven density (and thus, dielectric constant), the modulus of thissample of the ethylene glycol dried gel is an order of magnitude higherthan the conventional dried gel. The glycerol-based gels also seem tohave a high strength; generally, the strength is at least as good as theethylene glycol-based gels. The reasons for this strength increase areunclear but may be related to the very high surface area of these driedgels (>1,000 m² /g) and the seemingly narrow pore size distribution.

High surface area--We measured the surface areas of some dried bulkgels. These surface areas were on the order of 1,000 m² /g, as comparedto our typical dried gels which have surface area in the 600-800 m² /grange. These higher surface areas may imply smaller pore size andimproved mechanical properties. It is unclear at this time why thesehigher surface areas are obtained with the polyol-based -based driedgels.

Pore size distribution--The optical clarity of these dried bulk gels wasgreater than any dried gels at this density that we have previouslymade. It is possible that this excellent optical clarity is due to avery narrow pore size distribution. However, it is unclear why thepolyols have this affect. It is still not clear whether the apparentlynarrow pore size distribution is a result of a different microstructureat the gelation stage or differences in aging. Preliminary measurementson a bulk gel sample (with a density of about 0.22 g/cm³) showed thatthe mean pore diameter was 16.8 nm.

As shown above, some properties of the polyol-based gels apply to bothbulk gels and thin films. However, some advantages are most evident whenapplied to thin films, such as nanoporous dielectric films onsemiconductor wafers. One important advantage is that this new methodallows high quality nanoporous films to be processed with no atmosphericcontrols during deposition or gelation.

Although it is important to be able to deposit and gel thin nanoporousfilms without atmospheric controls, it is also desirable to age thinnanoporous films without atmospheric controls. It has been discoveredthat this presents a bigger challenge than deposition. The primaryreason is that while deposition and room temperature gelation can takeplace in minutes, or even seconds; room temperature aging typicallyrequires hours. Thus, an evaporation rate that provides acceptableshrinkage for a short process, may cause unacceptable shrinkage when theprocess times are lengthened by an order of magnitude.

As an example, we have found that with some polyol-based gels, such asthe ethylene glycol- and glycerol-based gels, a satisfactory aging timeat room temperature is on the order of a day. However, Table 2 showsthat, by using higher temperatures, we can age with times on the orderof minutes. Thus, when these times and temperatures are combined withthe evaporation rates of FIG. 1, FIG. 5, and FIG. 11, they give theapproximate thickness loss during aging as shown in Table 3. Theseestimated thickness losses need to be compared with acceptable thicknesslosses. While no firm guidelines for acceptable thickness loss exist,one proposed guideline, for some microcircuit applications such asnanoporous dielectrics, is that the thickness losses should be less than2% of the film thickness. For a hypothetical nominal film thickness of 1μm (Actual film thicknesses may typically vary from significantly lessthan 0.5 μm to several μm thick), this gives an allowable thickness lossof 20 nm. As shown in Table 3, the glycerol-based gels (and otherpolyol-based gels with low vapor pressures) can achieve this preliminarygoal without atmospheric control at room temperature. Thus, thisinvention allows thin film aerogels to be deposited, gelled, aged, anddried without atmospheric controls.

                  TABLE 2                                                         ______________________________________                                        Approximate Aging Time as a Function of Temperature For Some                  Polyol-Based Gels                                                             Aging Temperature                                                                          Aging Time For Polyol-Based Gels                                 (Degrees C.) (Order of Magnitude Approximations)                              ______________________________________                                        25           1 day                                                            100          5 minutes                                                        140          1 minute                                                         ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________    Approximate Thickness Loss During Aging vs. Saturation Ratio.                 Thickness Loss During Aging                                                         Ethanol-Based Gel                                                                         EG-Based Gel                                                                              Glycerol-Based Gel                              Aging Time/                                                                         % Saturation                                                                              % Saturation                                                                              % Saturation                                    Temperature                                                                         0%  50% 99% 0%  50% 99% 0%  50% 99%                                     __________________________________________________________________________    1 day/                                                                              8   7   86  17  7   172  13 5   .1                                      25° C.                                                                       mm  mm  μm                                                                             μm                                                                             μm                                                                             nm  nm  nm  nm                                      300 sec/                                                                            --  --  --   3  1.2  90 600 420  9                                      100° C.    μm                                                                             μm                                                                             nm  nm  nm  nm                                      60 sec/                                                                             --  --  --  --  --  --   6  3   60                                      140° C.                μm                                                                             μm                                                                             nm                                      __________________________________________________________________________

By using passive atmospheric control, this invention can be extended tohave even lower evaporation losses. This passive control involvesplacing the gel in a relatively small closed container, at least duringaging. In this aspect of the invention, evaporation from the wafer actsto raise the saturation ratio of the atmosphere inside the closedcontainer. At any given temperature, this evaporation continues untilthe partial pressure of the vapor increases enough to equal the vaporpressure of the liquid. Thus, solvent/temperature combinations withlower vapor pressure will not allow as much liquid solvent to evaporateas a higher vapor pressure combination allows. FIG. 12 shows how vaporpressure varies with temperature for several solvents. If the containersize is known, the amount of evaporation can be calculated. FIG. 13shows an estimate of how thick of layer of solvent could potentially beevaporated if a 70% porous gel is placed in a 5 mm high cylindricalcontainer that is the same diameter as the wafer. FIG. 3 shows a similarestimate for a container with a 1 mm high airspace above the wafer.These figures show that, with a 5 mm high airspace, the 20 nmpreliminary goal is feasible up to 50 degrees C. for ethyleneglycol-based gels and up to 120 degrees C. for glycerol-based gels. Withthe 1 mm airspace, the 20 nm goal is feasible up to 80 degrees C. forthe ethylene glycol-based gels and 150 degrees C. for the glycerol-basedgels. Of course, lower temperature processing allows less evaporation.Passive evaporation control using the 1 mm containers allows less than 1nm of thickness loss for both ethylene glycol-based and glycerol-basedgels at 20 degrees C.

There are many variations on this passive control approach. Onevariation allows the container size to increase. The thickness loss willlinearly increase with the container volume. However, even a 1000 cubiccentimeter container typically allows only 20 nm of ethylene glycolevaporation at 20 degrees C. Another variation is the gel porosity.Higher porosity gels generally experience greater thickness losses whilelower porosity gels generally experience slightly smaller thicknesslosses. Other polyols may be used. However, different polyols may havedifferent vapor pressure characteristics; thus, they may have differentthickness losses.

One disadvantage of polyols, especially trihydric alcohols and otherhigher viscosity polyols, are their relatively high viscosities whichcould cause problems with gap-filling and/or planarization. As describedin copending U.S. patent application serial #TBD (Attorney's DocketTI-21623), titled Aerogel Thin Film Formation From Multi-SolventSystems, by Smith et al., a low viscosity, high volatility solvent canbe used to lower the viscosity. We have compared the calculatedviscosity of some ethylene glycol/alcohol and glycerol/alcohol mixturesat room temperature. This comparison shows, small quantities of alcoholsignificantly reduces the viscosity of these mixtures. Also, if theviscosity using ethanol in the stock solution is higher than desired,further improvement can be realized by employing methanol andtetramethoxysilane in the precursor solution. The viscosities in ourcomparison were for pure fluid mixtures only. In fact, depending uponthe film precursor solution precursor solution might contain glycerol,alcohol, water, acid and partially reacted metal alkoxides. Afterrefluxing, but before catalysis, the measured viscosity as a function ofethylene glycol content is shown in Table 4. As predicted, the use ofmethanol significantly lowers the viscosity. Of course, the viscositycan be increased before deposition by catalyzing the condensationreaction and hence, the values reported in Table 4 represent lowerbounds.

                  TABLE 4                                                         ______________________________________                                        Measured Viscosity and Density of Glycol-Based Stock Solutions                Before Activation.                                                                             Viscosity                                                                     @25° C.                                                                         Viscosity @40° C.                            Stock Solution "Solvent"                                                                       (cp)     (cp)                                                ______________________________________                                        100% EtOH        1.5      --                                                  40% Ethylene glycol/60% EtOH                                                                   3.1      --                                                  49% Ethylene glycol/51% EtOH                                                                   4.0      --                                                  60% Ethylene glycol/40% EtOH                                                                   5.4      --                                                  40% Ethylene glycol/60%                                                                        1.6      --                                                  Methanol                                                                      100% Ethylene glycol                                                                           11.0     7.8                                                 40% Glycerol/60% EtOH                                                                          5.8      --                                                  50% Glycerol/50% EtOH                                                                          9.0      --                                                  60% Glycerol/40% EtOH                                                                          15.5     --                                                  100% Glycerol    1000.    7.8                                                 ______________________________________                                    

This multi-solvent approach may be combined with or replaced by analternative approach. This alternate approach use elevated temperaturesto reduce the sol viscosity during application. For example, themeasured viscosity of the TEOS/ethylene glycol/water/nitric acidprecursor described in the second preferred embodiment is 11 centipoise(cp) at 25 degrees C., but only 7.8 cp at 40 degrees C. Thus by heatingand/or diluting the precursor during deposition, (such as by heating thetransfer line and deposition nozzle of a wafer spin station) theviscosity of the precursor sol can be lowered to nearly any givenviscosity. Not only does this preheat lower the sol viscosity, it mayalso speed gel times and accelerate the evaporation of any highvolatility solvents. It may also be desirable to preheat the wafer. Thiswafer preheat should improve process control and may improve gap fill,particularly for the more viscous precursors. However, for manyapplications, wafer preheat is not required, thus simplifying processflows. When using a spin-on application method with this no waferpreheat approach, the spin station would not require a temperaturecontrolled spinner.

Dried gels produced with this simple thin film aerogel fabricationprocess can be used in many applications. Some of these uses may nothave been cost effective using prior art methods. These uses include lowdielectric constant thin films (particularly on semiconductorsubstrates), miniaturized chemical sensors, thermal isolationstructures, and thermal isolation layers (including thermal isolationstructures for infrared detectors). As a general rule, many lowdielectric constant thin films prefer porosities greater than 60%, withcritical applications preferring porosities greater than 80 or 90%, thusgiving a substantial reduction in dielectric constant. However,structural strength and integrity considerations may limit the practicalporosity to no more than 90%. Some applications, including thermalisolation structures and thermal isolation layers, may need to sacrificesome porosity for higher strength and stiffness. These higher stiffnessrequirements may require dielectrics with porosities as low as 30 or45%. In other high strength/toughness applications, especially sensors,where surface area may be more important than density, it may bepreferable to use a low porosity gel with a porosity between 15% and40%.

A method for forming a thin film nanoporous dielectric on asemiconductor substrate is disclosed herein. This method comprises thesteps of providing a semiconductor substrate and depositing an aerogelprecursor sol upon the substrate. This aerogel precursor sol comprises ametal-based aerogel precursor reactant and a first solvent comprising afirst polyol; wherein, the molar ratio of the first solvent molecules tothe metal atoms in the reactant is at least 1:16. The method furthercomprises allowing the deposited sol to create a gel, wherein the gelcomprises a porous solid and a pore fluid; and forming a dry, nanoporousdielectric by removing the pore fluid in a drying atmosphere withoutsubstantially collapsing the porous solid.

Preferably, the first polyol is glycerol. Preferably, the aerogelprecursor reactant may selected from the group consisting of metalalkoxides, at least partially hydrolyzed metal alkoxides, particulatemetal oxides, and combinations thereof. Typically, the molar ratio ofthe first solvent molecules to the metal atoms in the reactant is nogreater than 12:1, and preferably, the molar ratio of the first solventmolecules to the metal atoms in the reactant is between 1:2 and 12:1. Insome embodiments, the molar ratio of the first solvent molecules to themetal atoms in the reactant is between 2.5:1 and 12:1. In this method,it is also preferable that the nanoporous dielectric has a porositygreater than 60% and an average pore diameter less than 25 nm. It isfurther preferred that the pressure of the drying atmosphere during theforming step is less than the critical pressure of the pore fluid. Insome embodiments, the aerogel precursor also comprises a second solvent.Preferably, the second solvent has a boiling point lower thanglycerol's. In some embodiments, the first solvent also comprises aglycol, preferably selected from the group consisting of ethyleneglycol, 1,4-butylene glycol, 1,5-pentanediol, and combinations thereof.After aging but before drying, in some embodiments, (especially in someglycerol-based mixtures) the aging solvent is replaced by a dryingfluid. This allows, e.g. rapid, lower temperature (e.g. roomtemperature) drying with a fluid that evaporates faster and has asuitably low surface tension. Examples of drying fluids include,ethanol, acetone, 2-ethylbutyl alcohol and some alcohol-water mixtures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, including various features and advantagesthereof, may be best understood with reference to the followingdrawings, wherein:

FIG. 1 contains a graph of the variation of evaporation rate withsaturation ratio and solvent type.

FIG. 2 contains a graph of the change in gel times (without solventevaporation) for bulk ethylene glycol-based gels as a function of basecatalyst

FIG. 3 contains a graph showing the shrinkage of a thin film when driedin a 1 mm thick container.

FIG. 4 contains a graph of the theoretical molar ratio of glycerolmolecules to metal atoms vs. porosity of a nanoporous dielectricaccording to the present invention.

FIG. 5 contains a graph of the evaporation rate for ethylene glycol as afunction of temperature and atmospheric saturation ratio.

FIG. 6 contains a graph of the variation in refractive index duringprocessing for a film produced using a 60/40 ethylene glycol/ethanolsolution with a substantially uncontrolled atmosphere.

FIG. 7 contains a graph of the theoretical relationship betweenporosity, refractive index, and dielectric constant for nanoporoussilica dielectrics.

FIGS. 8A-8B contain cross-sections of a semiconductor substrate atseveral points during deposition of a thin film according to the presentinvention.

FIG. 9 is a flow chart of a deposition process for a nanoporousdielectric according to the present invention.

FIG. 10 contains a graph of the theoretical molar ratio of first solventmolecules to metal atoms vs. porosity of a nanoporous dielectricaccording to the present invention.

FIG. 11 contains a graph of the evaporation rate for glycerol a functionof temperature and atmospheric saturation ratio.

FIG. 12 contains a graph showing the change in vapor pressure withtemperature.

FIG. 13 contains a graph showing the shrinkage of a thin film when driedin a 5 mm thick container.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Typical sol-gel thin film processes produce gels which collapse anddensify upon drying, thus forming xerogels having limited porosity (Upto 60% with large pore sizes, but generally substantially less than 50%with pore sizes of interest). Under the uncontrolled drying conditionsof xerogel film formation, many of the internal pores permanentlycollapse. However, in thin film aerogel formation, the pores remainsubstantially uncollapsed, even though there is often some shrinkageduring aging and/or drying that affects the final density.

Referring now to FIG. 8A, a semiconductor substrate 10 (typically inwafer form) is shown. Common substrates include silicon, germanium, andgallium arsenide, and the substrate may include active devices, lowerlevel wiring and insulation layers, and many other common structures notshown but known to those skilled in the art. Several patternedconductors 12 (e.g., of an Al-0.5%Cu composition) are shown on substrate10. Conductors 12 typically run parallel for at least part of theirlength, such that they are separated by gaps 13 of a predetermined width(typically a fraction of a micron). Both the conductors and gaps mayhave height-to-width ratios much greater than shown, with larger ratiostypically found in devices with smaller feature sizes.

In accordance with a first embodiment of the present invention, mix 61.0mL tetraethylorthosilicate (TEOS), 61.0 mL glycerol, 4.87 mL water, and0.2 mL 1M HNO₃ and reflux for 1.5 hours at ˜60° C. After the mixture isallowed to cool, the solution may be diluted down with ethanol to acomposition of 80% (by volume) original stock solution and 20% (byvolume) ethanol, thus reducing the viscosity. This is mixed vigorouslyand typically stored in a refrigerator at ˜7° C. to maintain stabilityuntil use. The solution is warmed to room temperature prior to filmdeposition. 3-5 mL of this precursor sol may be dispensed at roomtemperature onto substrate 10, which is then spun at 1500 to 5000 rpm(depending on desired film thickness) for about 5-10 seconds to form solthin film 14. The deposition can be performed in an atmosphere that hasno special control of solvent saturation (e.g., in a cleanroom withstandard humidity controls). During and after this deposition andspinning, the ethanol/water azeotropic mixture is evaporating from film14, but due to glycerol's low volatility, no substantial evaporation ofthe glycerol is occurring. This evaporation shrinks thin film 14 andconcentrates the silica content of the sol forming reduced thicknessfilm 18. FIG. 8B shows a reduced thickness sol film 18 obtained aftersubstantially all (about 95% or more) of the ethanol has been removed.This concentrating typically causes gelation within minutes or seconds.

Film 18 has an approximately known ratio of silicon to pore fluid at thegel point. This ratio is approximately equal to the ratio of TEOS toglycerol in the as-deposited sol (with minor changes due to remainingwater, continued reactions and incidental evaporation). To the extentthat the gel is prevented from collapsing, this ratio will determine thedensity of the aerogel film that will be produced from the sol thinfilm.

After gelation, the thin film wet gel 18 comprises a porous solid and apore fluid, and can preferably be allowed time to age at one or morecontrolled temperatures, e.g., about a day at room temperature. Itshould be noted that the pore fluid changes somewhat during processing.These changes may be due to continued reactions,evaporation/condensation, or chemical additions to the thin film. Aftergelation, the pore fluid may preferably be left in place, however, itmay be diluted or replaced by a different fluid (e.g. replace glyceroland water mixture with glycerol). Whether this fluid is identical to theas-gelled fluid or not, the pore fluid that is present during aging issometimes referred to as "aging fluid". Aging may preferably beaccomplished by letting the substrate and gel sit for approximately 24hours at about 25 degrees C. or by heating it to 130°-150° C. for about1 minute in a closed container.

Aged film 18 may be dried without substantial densification by one ofseveral methods, including supercritical fluid extraction. However, withthese new glycerol-based gels, one alternative is to use a solventexchange to replace the aging fluid with a drying fluid and then air drythe film 18 from this drying fluid. This preferred drying method uses asolvent exchange to dilute the aging fluid or replace it with adifferent fluid (e.g. use a large volume of acetone to dilute theglycerol and water mixture, thus forming a mixture dominated byacetone). Whether this fluid is identical to the aging fluid or not, thepore fluid that is present during drying is sometimes referred to as"drying fluid". If used, the solvent exchange replaces the aging fluidthat is dominated by the glycerol and its associated high surfacetension with a drying fluid that has a lower surface tension. Thissolvent exchange may preferably be carried out by dispensingapproximately 5-8 mL of acetone at room temperature onto aged thin film18, then spinning the wafer between approximately 250 and 500 rpm forabout 5-10 seconds. In this solvent exchange method, it is preferred toremove nearly all the glycerol before drying. The drying fluid (acetonein this case) is finally allowed to evaporate from the wet gel 18,forming a dry porous dielectric (dried gel).

This evaporation be performed by exposing the wafer surface to anatmosphere that is not near saturated with the drying fluid. Forexample, the wafer could be in a substantially uncontrolled atmosphere,or a drying gas could be introduced into the atmosphere. To preventboiling, drying should start at a temperature somewhat below the boilingpoint of the drying fluid, such as room temperature. If a higher boilingpoint drying fluid, such as ethylene glycol, is used, the startingdrying temperature can be increased to a temperature near or equal tothe aging temperature. As the thin film becomes predominately dry(typically within seconds), the temperature should then be increasedabove the boiling point of both the aging fluid and the drying fluid.This method prevents destructive boiling, yet insures that all fluid isremoved.

In order to reduce the dielectric constant, it is preferable todehydroxylate (anneal) the dried gel. This may be done by placing thewafer in a forming gas atmosphere comprised of 10 volume % H2, 90 volume% N2 at atmospheric pressure, and baking at 450° C. for approximately 30minutes.

FIG. 9 contains a flow chart of a general method for obtaining anaerogel thin film from a precursor sol according to one embodiment ofthe present invention. Table 5 is a quick summary of some of thesubstances used in this method.

                  TABLE 5                                                         ______________________________________                                        Substance Summary                                                                            Functional                                                     Ref  Specific  Descrip-                                                       #    Example   tion      Preferred Alternates                                 ______________________________________                                        10   Silicon   Semi-     Ge, GaAs, active devices, lower level                               conductor layers                                                              Substrate                                                      12   Al-0.5%Cu Patterned Al, Cu, other metals, polysilicon                                   Con-                                                                          ductors                                                             TEOS      Precursor Other silicon-based metal alkoxides                                 Sol       (TMOS, MTEOS, BTMSE, etc.),                                         Reactant  alkoxides of other metals, particulate                                        metal oxides, organic precursors, and                                         combinations thereof                                      Glycerol  Precursor Other polyols, combinations of                                      Sol       glycerol, Ethylene glycol,                                          First     1,4-butylene glycol, 1,5-pentanediol,                               Solvent   and/or other polyols.                                               (Low                                                                          volatility)                                                         Nitric Acid                                                                             Precursor Other acids                                               (HNO.sub.3)                                                                             Sol                                                                           Stabilizer                                                          Ethanol   Precursor Methanol, other alcohols                                            Sol                                                                           Second                                                                        Solvent                                                                       (High                                                                         volatility)                                                         Ethanol   Viscosity Methanol, other alcohols                                            Reduction                                                                     Solvent                                                             TMCS      Surface   Hexamethyldisilazane (HMDS),                                        Modification                                                                            trimethylmethoxysilane,                                             Agent     dimethyldimethoxysilane, phenyl                                               compounds and fluorocarbon                                                    compounds.                                                Ammonium  Gelation  Ammonia, volatile amine species,                          Hydroxide Catalyst  volatile fluorine species, and other                      (NH.sub.4 OH)       compounds that will raise the pH of                                           the deposited sol. Nitric acid and                                            other compounds that will lower the                                           pH.                                                       As-Gelled Aging Fluid                                                                             Glycerol, ethylene glycol, other                          Pore Fluid          polyols, water, ethanol, other                                                alcohols, combinations thereof.                           Acetone   Drying Fluid                                                                            Aging fluid, heated aging fluid,                                              heptane, isoproponal, ethanol,                                                methanol, 2-ethylbutyl alcohol,                                               alcohol/water mixtures, ethylene                                              glycol, other liquids that are miscible                                       with the aging fluid, yet have lower                                          surface tension than the aging fluid,                                         combinations thereof.                                ______________________________________                                    

In accordance with a second, more TEOS-rich, embodiment of the presentinvention, 140 mL tetraethylorthosilicate (TEOS), 61.0 mL glycerol, 12.0mL water, 140 mL ethanol, and 0.352 mL 1M HNO₃ are mixed and reflux for1.5 hours at ˜60° C. After the mixture is allowed to cool, the solutionmay be diluted down with ethanol to a composition of 80% (by volume)original stock solution and 20% (by volume) ethanol, thus reducing theviscosity. This is typically stored in a refrigerator at ˜7° C. tomaintain stability until use. The solution is warmed to room temperatureprior to film deposition. 3-5 mL of this precursor sol may be dispensedat room temperature onto substrate 10, which is then spun at 1500 to5000 rpm (depending on desired film thickness) for about 5-10 seconds toform sol thin film 14. The deposition can be performed in an atmospherethat is not solvent controlled (e.g. standard exhausts in a cleanroomwith standard humidity controls). During and after this deposition andspinning, ethanol (a reaction product from the TEOS and water) and wateris evaporating from film 14, but due to glycerol's low volatility, nosubstantial evaporation of the glycerol is occurring. This evaporationshrinks thin film 14 and concentrates the silica content of the solforming reduced thickness film 18. FIG. 8B shows a reduced thickness solfilm 18 obtained after substantially all (about 95% or more) of thewater has been removed. This concentrating typically causes gelationwithin minutes.

Further processing generally follow the process described in the firstpreferred embodiment. After gelation, the thin film wet gel 18 comprisesa porous solid and a pore fluid, and can preferably be allowed time toage at one or more controlled temperatures. Aging may preferably beaccomplished by letting the device sit for approximately 24 hours at 25degrees C. Aged film 18 may be dried without substantial densificationby one of several methods, including supercritical fluid extraction.However, with these new glycerol-based gels, it is preferable to performa solvent exchange followed by air drying the film 18 from the dryingfluid, as described in the first preferred embodiment. In order toreduce the dielectric constant, it is preferable to dehydroxylate(anneal) the dried gel, as described in the first preferred embodiment.

Other ratios of solvent to reactant ratios can be used to providedifferent porosities. FIG. 4 shows the theoretical relationship betweenthe molar ratio of glycerol molecules to metal atoms and the porosity ofa nanoporous dielectric for the case where all ethanol is evaporatedfrom the deposited sol. However, the lower porosity gels require care toprevent early gelation. This may comprise pH adjustment, temperaturecontrol, or other methods known in the art. In some applications, it isalso permissible to allow ethanol evaporation after gelation.

In accordance with a ethylene glycol containing third embodiment of thepresent invention, mix tetraethylorthosilicate (TEOS), ethylene glycol,ethanol, water, and acid (1M HNO₃) in a molar ratio of 1:2.4:1.5:1:0.042and reflux for 1.5 hours at ˜60° C. After the mixture is allowed tocool, the solution is diluted down with ethanol to a composition of 70%(by volume) original stock solution and 30% (by volume) ethanol. This ismixed vigorously and typically stored in a refrigerator at ˜7° C. tomaintain stability until use. The solution is warmed to room temperatureprior to film deposition. A mixture of stock solution and 0.25M NH₄ OHcatalyst (10:1 volume ratio) is combined and mixed. 3-5 mL of thisprecursor sol may be dispensed at room temperature onto substrate 10,which is then spun at 1500 to 5000 rpm (depending on desired filmthickness) for about 5-10 seconds to form sol thin film 14. Thedeposition can be performed in a cleanroom with standard humiditycontrols. During and after this deposition and spinning, theethanol/water azeotropic mixture is evaporating from film 14, but due toethylene glycol's low volatility, no substantial evaporation of theethylene glycol is occurring. This evaporation shrinks thin film 14 andconcentrates the silica content of the sol forming reduced thicknessfilm 18. FIG. 8B shows a reduced thickness sol film 18 obtained aftersubstantially all (about 95% or more) of the ethanol has been removed.This concentrating, combined with the catalyst, typically causesgelation within minutes or seconds.

Film 18 has an approximately known ratio of silicon to pore fluid at thegel point. This ratio is approximately equal to the ratio of TEOS toethylene glycol in the as-deposited sol (with minor changes due toremaining water, continued reactions and incidental evaporation). To theextent that the gel is prevented from collapsing, this ratio willdetermine the density of the aerogel film that will be produced from thesol thin film.

After gelation, the thin film wet gel 18 comprises a porous solid and apore fluid, and can preferably be allowed time to age at one or morecontrolled temperatures, e.g., about a day at room temperature. Itshould be noted that the pore fluid changes somewhat during processing.These changes may be due to continued reactions,evaporation/condensation, or chemical additions to the thin film. Aftergelation, the pore fluid may preferably be left in place, however, itmay be diluted or replaced by a different fluid (e.g. replace ethyleneglycol and water mixture with ethylene glycol). Whether this fluid isidentical to the as-gelled fluid or not, the pore fluid that is presentduring aging is sometimes referred to as "aging fluid". Aging maypreferably be accomplished by letting the device sit in a near saturatedaging fluid atmosphere for approximately 5 minutes at about 100 degreesC. It is preferred that the thin film wet gel 18 does not dryprematurely. However, it is also preferred to prevent condensation onthe surface of the wet gel. This balance between preventing both dryingand condensation is a motivation behind the preference for maintainingthe near saturated aging fluid atmosphere. An acceptable aging approachat 100 degrees C. may be to control the ethylene glycol content of anaging atmosphere to 99±1% of saturation.

Aged film 18 may be dried without substantial densification by one ofseveral methods, including supercritical fluid extraction. However, withthese new ethylene glycol-based gels, it is preferable to air dry thefilm 18 from the aging fluid. This air drying may be performed byexposing the wafer surface to an atmosphere that is not near saturatedwith the drying fluid. For example, the wafer could be moved from theaging atmosphere to a substantially uncontrolled atmosphere, or anothergas could be introduced into the aging atmosphere. To prevent boiling,drying should start at a temperature somewhat below the boiling point ofthe drying fluid, such as room temperature or the aging temperature. Asthe thin film becomes predominately dry (typically within seconds), thetemperature may then be increased above the boiling point of both theaging fluid and the drying fluid. This method prevents destructiveboiling, yet insures that all fluid is removed.

It should be noted that the aging fluid (somewhat like the pore fluid)changes somewhat during processing. These changes may be due tocontinued reactions, evaporation/condensation, or chemical additions tothe thin film. Preferably, the aging fluid may be left in place afteraging. However, one may use a solvent exchange to dilute the aging fluidor replace it with a different fluid (e.g. use a large volume of acetoneto dilute the ethylene glycol and water mixture, thus forming a mixturedominated by acetone). Whether this fluid is identical to the agingfluid or not, the pore fluid that is present during drying is sometimesreferred to as "drying fluid". If used, The solvent exchange replacesthe aging fluid that is dominated by the ethylene glycol and itsassociated high surface tension with a drying fluid that has a lowersurface tension. This solvent exchange may preferably be carried out bydispensing approximately 5-8 mL of acetone at room temperature onto agedthin film 18, while spinning the wafer between approximately 250 and 500rpm for about 5-10 seconds. In this solvent exchange method, it ispreferred to remove nearly all the ethylene glycol before drying. Thedrying fluid (acetone in this case) is finally allowed to evaporate fromthe wet gel 18, forming a dry porous dielectric (dried gel).

In order to reduce the dielectric constant, it is preferable todehydroxylate (anneal) the dried gel. This may be done by placing thewafer in a forming gas atmosphere comprised of 10 volume % H2, 90 volume% N2 at atmospheric pressure, and baking at 450° C. for approximately 30minutes.

In accordance with a fourth, more ethylene glycol rich, embodiment ofthe present invention, mix tetraethylorthosilicate (TEOS), ethyleneglycol, water, and acid (1M HNO₃) in a molar ratio of 1:4:1:0.042 andreflux for 1.5 hours at ˜60° C. After the mixture is allowed to cool,the solution may be diluted down with ethanol to a composition of 70%(by volume) original stock solution and 30% (by volume) ethanol, thusreducing the viscosity. This is typically stored in a refrigerator at˜7° C. to maintain stability until use. The solution is warmed to roomtemperature prior to film deposition. 3-5 mL of this precursor sol maybe dispensed (without catalyst) at room temperature onto substrate 10,which is then spun at 1500 to 5000 rpm (depending on desired filmthickness) for about 5-10 seconds to form sol thin film 14. Thedeposition can be performed in a cleanroom with standard exhausts andhumidity controls. During and after this deposition and spinning,ethanol (a reaction product from the TEOS and water) and water isevaporating from film 14, but due to ethylene glycol's low volatility,no substantial evaporation of the ethylene glycol is occurring. Thisevaporation shrinks thin film 14 and concentrates the silica content ofthe sol forming reduced thickness film 18. FIG. 8B shows a reducedthickness sol film 18 obtained after substantially all (about 95% ormore) of the water has been removed. This concentrating typically causesgelation within minutes.

Further processing generally follow the process described in the thirdpreferred embodiment. After gelation, the thin film wet gel 18 comprisesa porous solid and a pore fluid, and can preferably be allowed time toage at one or more controlled temperatures. Aging may preferably beaccomplished by letting the device sit in a near saturated aging fluidatmosphere for approximately 5 minutes at about 100 degrees C. Aged film18 may be dried without substantial densification by one of severalmethods, including supercritical fluid extraction. However, with thesenew ethylene glycol-based gels, it is preferable to air dry the film 18from the aging fluid, as described in the third preferred embodiment. Inorder to reduce the dielectric constant, it is preferable todehydroxylate (anneal) the dried gel, as described in the thirdpreferred embodiment.

An alternate method utilizes aging and drying glycerol films withoutsolvent-exchange. An unaged wafer is placed in a small volume furnace,and the furnace preferably evacuated at room temperature. The furnaceremains sealed as the temperature is ramped up, aging the film. Afterthe drying temperature at which the glycerol viscosity is low enough(compared to the strength of an aged film of the predeterminedporosity), the glycerol in the furnace atmosphere is removed. Note thatthe drying temperature, in many applications, is greater than theboiling point of glycerol, in which cases; the furnace needs towithstand the pressure and care needs to be taken that the glycerol inthe furnace atmosphere is, especially at first, slowly removed. Theglycerol in the furnace atmosphere may, e.g., be removed by bleeding offthe pressure, by a vacuum pump, or by sweeping the glycerol off with theannealing gas (e.g. forming gas). The furnace temperature may be heldconstant or continue to be raised while the glycerol is being removed(the furnace may be ramped on up to annealing temperature while sweepingthe glycerol off with the annealing gas). While some glycerol can beintroduced during heating to minimize evaporation from the film,preferably the furnace volume is low enough that evaporation does notsignificantly reduce film thickness even without the introduction ofglycerol during heating.

Other ratios of solvent to reactant ratios can be used to providedifferent porosities. FIG. 10 shows the theoretical relationship betweenthe molar ratio of ethylene glycol molecules to metal atoms and theporosity of a nanoporous dielectric for the case where all ethanol isevaporated from the deposited sol. However, the lower porosity gels mayrequire care to prevent early gelation. This may comprise pH adjustment,temperature control, or other methods known in the art. In someapplications, it is also permissible to allow ethanol evaporation aftergelation.

Although this invention has been described in terms of severalembodiments, many of these steps may be modified within the scope of theinvention, and other steps can be included to enhance the overallprocess. For example, the initial thin film may be deposited by othercommon methods, such as dip-coating or spray-coating instead ofspin-coating. Likewise, the solvent exchange may use dip coating, spraycoating, or immersion in a liquid or vaporous solvent instead ofspin-coating. When using a vaporous solvent, the wafer may be cooled toa temperature lower than the atmosphere, thus promoting condensation onthe wafer. While water might be considered a solvent in such a process,for discussion purposes in this application, water is not considered asolvent.

One modification is to change the mix ratios of polyol and alcohol ofthe first preferred embodiment. This variation can change the gel'sproperties. One such change is the gel time. Table 6 below shows theresults of varying the ethanol to ethylene glycol ratios in theprecursor sol of some sample bulk gels with catalysts. These gels usedgenerally the same sol mixture as the third preferred embodiment exceptfor the ethanol to ethylene glycol ratio. Also, in the non-polyol-basedmix, the catalyst concentration is different. This non-polyol-based gelused 0.5M NH₄ OH catalyst in a volume ratio of 1:10, instead of the0.25M NH₄ OH used in the others.

                  TABLE 6                                                         ______________________________________                                        Effect of Varying the Ethylene Glycol Content of the Precursor Sol                         Ethanol   Ethylene Glycol                                                     Content   Content     Gel Time                                   Bulk Exam #  (mL)      (mL)        (minutes)                                  ______________________________________                                        1            61        0            7 to 10                                   (Non-Polyol-Based)                                                            2            36.6      24.4        5 to 7                                     3            30.5      30.5        2 to 3                                     4            24.4      36.6        1 to 2                                     5            0         61          1 to 2                                     ______________________________________                                    

Another example of modification to the basic method is that, beforedrying (and typically, but not necessarily, after aging), the thin filmwet gel 18 may have its pore surfaces modified with a surfacemodification agent. This surface modification step replaces asubstantial number of the molecules on the pore walls with those ofanother species. If a surface modifier is applied, it is preferable toremove the water from the wet gel 18 before the surface modifier isadded. The water can be removed by immersing the wafer in pure ethanol,preferably by a low speed spin coating as described in the solventexchange in the first embodiment example. This water removal could bebeneficial, because water will react with many surface modificationagents, such as TMCS; however, it is not necessary. With our newpolyol-based method, surface modification need not be performed to helplessen pore collapse, it can be used to impart other desirableproperties to the dried gel. Some examples of potentially desirableproperties are hydrophobicity, reduced dielectric constant, increasedresistance to certain chemicals, and improved temperature stability.Some potential surface modifiers that may impart desirable propertiesinclude hexamethyl-disilazane (HMDS), the alkyl chlorosilanes(trimethylchlorosilane (TMCS), dimethyl-dichlorosilane, etc.), thealkylalkoxysilanes (trimethylmethoxysilane, dimethyl-dimethoxysilane,etc.), phenyl compounds and fluorocarbon compounds. The useful phenylcompounds will typically follow the basic formula, Ph_(x) A_(y)SiB.sub.(4-x-y), where, Ph is a phenolic group, A is a reactive groupsuch as Cl or OCH₃, and B are the remaining ligands which, if there aretwo, can be the same group or different. Some examples of these phenylsurface modification agents include compounds with 1 phenolic group suchas phenyltrichlorosilane, phenyltrifluorosilane, phenyltrimethoxysilane,phenyltriethoxysilane, phenylmethylchlorosilane,phenylethyldichlorosilane, phenyldimethylethoxysilane,phenyldimethylchlorosilane, phenyidichlorosilane,phenyl(3-chloropropyl)dichlorosilane, phenylmethylvinylchlorosilane,phenethyidimethylchlorosilane, phenyltrichlorosilane,phenyltrimethoxysilane, phenyltris(trimethylsiloxy)silane, andphenylallyidichlorosilane. Other examples of these phenyl surfacemodification agents include compounds with 2 phenolic groups such asdiphenyldichlorosilane, diphenylchlorosilane, diphenylfluorosilane,diphenylmethylchiorosilane, diphenylethylchlorosilane,diphenyidimethoxysilane, diphenylmethoxysilane, diphenylethoxysilane,diphenylmethylmethoxysilane, diphenylmethylethoxysilane anddiphenyldiethoxysilane. These phenyl surface modification agents alsoinclude compounds with 3 phenolic groups such as triphenylchlorosilane,triphenylflourosilane, and triphenylethoxysilane. Another importantphenyl compound, 1,3-diphenyltetramethyldisilazane, is an exception tothis basic formula. These lists are not exhaustive, but do convey thebasic nature of the group. The useful fluorocarbon based surfacemodification agents include (3,3,3-trifluoropropyl)trimethoxysilane),(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1 dimethylchlorsilane, and otherfluorocarbon groups that have a reactive group, such as Cl or OCH₃, thatwill form covalent bonds with a hydroxyl group.

The paragraph above lists some of the typical useful properties for manyconventional applications. However, there are other potentialapplications for nanoporous dielectrics and aerogels that may havedifferent desirable properties. Examples of some other potentiallydesirable properties include hydrophilicity, increased electricalconductivity, increased dielectric breakdown voltage, increasedreactivity with certain chemicals, and increased volatility. This listis not exhaustive. However, it shows that, depending upon theapplication, many different types of properties may be desirable. Thus,it is clear that many other materials that will form covalent bonds withhydroxyl groups are potential surface modifiers that may impart otherpotentially desirable properties.

This invention also comprises using gelation catalysts with theglycerol-based and other polyol-based sols, not just the ethyleneglycol-based sols. This also includes the allowance of other gelationcatalysts in place of the ammonium hydroxide and/or for the gelationcatalyst to be added after deposition. Typically, these alternatecatalysts modify the pH of the sol. It is preferable to use catalyststhat raise the pH, although acid catalysts can be used. Typically, acidcatalysis results in longer processing times and a denser dielectricthan a base catalyzed process. Some examples of other preferred gelationcatalysts include ammonia, the volatile amine species (low molecularweight amines) and volatile fluorine species. When the catalyst is addedafter deposition, it is preferable to add the catalyst as a vapor, mist,or other vaporish form.

Thus, this invention allows production of nanoporous dielectrics at roomtemperature and atmospheric pressure, without a separate surfacemodification step. Although not required to prevent substantialdensification, this new method does not exclude the use of supercriticaldrying or surface modification steps prior to drying. To the extent thatthe freezing rates are fast enough to prevent large (e.g. 50 nm)crystals, it is also compatible with freeze drying. In general, this newmethod is compatible with most prior art aerogel techniques. Althoughthis new method allows fabrication of aerogels without substantial porecollapse during drying, there may be some permanent shrinkage duringaging and/or drying. This shrinkage mechanism is not well understood;however, it behaves in a manner similar to syneresis.

Other examples of modifications involve the reaction atmosphere and/ortemperature. Also coating and gelation need not be performed in the samechamber or even in the same atmosphere. For instance, coating may bedone with a controlled ambient that prevents evaporation of lowvolatility components (particularly at higher temperatures where eventhe low volatility components evaporate more rapidly), or in an ambientthat also prevents evaporation of high volatility components.Additionally, the substrate may have its temperature elevated to speedsurface modification and/or gelation. Also, total pressure and/ortemperature may be varied to further control evaporation rates and/orgel time. Elevated temperature processing is typically performed at noless than 40° C.; however, 50° C. is preferred, and 70° C. is morepreferred. When working at elevated temperatures, care should be taken(e.g., the partial pressures in the reaction atmosphere should be highenough) to prevent solvent boiling.

Although TEOS has been used as a representative example of a reactant,other metal alkoxides may be used either alone or in combination withTEOS or each other to form a silica network. These metal alkoxidesinclude tetramethylorthosilicate (TMOS), methyltriethoxysilane (MTEOS),1,2-Bis(trimethoxysilyl)ethane (BTMSE), combinations thereof, and othersilicon-based metal alkoxides known in the art. A sol may also be formedfrom alkoxides of other metals known in the art such as aluminum andtitanium. Some other precursor sols known in the art include particulatemetal oxides and organic precursors. Two representative particulatemetal oxides are pyrogenic (fumed) silica and colloidal silica. Somerepresentative organic precursors are melamine, phenol furfural, andresorcinol. In addition to alternate reactants, alternate solvents mayalso be used. Some examples of preferred alternates for ethanol aremethanol and the other higher alcohols. Other acids may be used as aprecursor sol stabilizer in place of the nitric acid.

An additional modification is to allow and/or promote the formation ofmoderate sized (15 to 150 monomers per molecule) oligomers in theprecursor sol. These larger oligomers may speed the gelation process inthe deposited sol. A sol containing large oligomers may have a higherviscosity than a sol with small oligomers. However, as long as theviscosity is stable, this higher viscosity can be compensated by methodsknown in the art, such as adjusting solvent ratios and spin conditions.To help achieve this desired stable viscosity, the oligomerization mayneed to be slowed or substantially halted before deposition. Potentialmethods of promoting oligomerization might include heating the precursorsol, evaporating solvent, or adding small amounts of a gelation catalystsuch as ammonium hydroxide. Potential methods of retardingoligomerization might include cooling the precursor sol, diluting thesol with a solvent, or restoring the precursor sol to a pH thatminimizes condensation and gelation (Nitric acid could be used inconjunction with the ammonium hydroxide exemplified above).

Although the present invention has been described with several sampleembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. A method for forming a thin film aerogel on asemiconductor substrate, the method comprising the steps of:a) providinga semiconductor substrate comprising a microelectronic circuit; b)depositing an aerogel precursor sol upon said substrate; wherein saidaerogel precursor sol comprisesa metal-based aerogel precursor reactant,wherein said reactant is a compound containing metal atoms, and a firstsolvent comprising glycerol; wherein,the molar ratio of said firstsolvent molecules to the metal atoms in said reactant is at least 1first solvent molecule per 16 metal atoms; c) allowing said depositedsol to create a gel, wherein said gel comprises a porous solid and apore fluid; and d) forming a dry aerogel by removing said pore fluid. 2.A method for forming a thin film aerogel on a semiconductor substrate,the method comprising the steps of:a) providing a semiconductorsubstrate comprising a microelectronic circuit; b) depositing an aerogelprecursor sol upon said substrate; wherein said aerogel precursor solcomprisesa metal-based aerogel precursor reactant, wherein said reactantis a compound containing metal atoms, and a first solvent comprising afirst polyol; wherein,said first polyol does not comprise a glycol;wherein,the molar ratio of said first solvent molecules to the metalatoms in said reactant is at least 1 first solvent molecule per 16 metalatoms; c) allowing said deposited sol to create a gel, wherein said gelcomprises a porous solid and a pore fluid; and d) forming a dry aerogelby removing said pore fluid.
 3. The method of claim 2, wherein:saidfirst solvent also comprises a second polyol.
 4. The method of claim 3,wherein:said second polyol comprises a glycol.
 5. A method for forming athin film nanoporous dielectric on a semiconductor substrate, the methodcomprising the steps of:a) providing a semiconductor substrate; b)depositing an aerogel precursor sol upon said substrate; wherein saidaerogel precursor sol comprisesa metal-based aerogel precursor reactant,wherein said reactant is a compound containing metal atoms, and a firstsolvent comprising glycerol; wherein,the molar ratio of said firstsolvent molecules to the metal atoms in said reactant is at least 1first solvent molecule per 16 metal atoms; c) allowing said depositedsol to create a gel, wherein said gel comprises a porous solid and apore fluid; and d) forming a dry, nanoporous dielectric by removing saidpore fluid.
 6. A method for forming a thin film nanoporous dielectric ona semiconductor substrate, the method comprising the steps of:a)providing a semiconductor substrate; b) depositing an aerogel precursorsol upon said substrate; wherein said aerogel precursor sol comprisesanaerogel precursor reactant selected from the group consisting of metalalkoxides, at least partially hydrolyzed metal alkoxides, andcombinations thereof, wherein said metal alkoxide contains metal atoms,and a first solvent comprising glycerol; wherein,the molar ratio of saidfirst solvent molecules to the metal atoms in said reactant is at least1 first solvent molecule per 16 metal atoms; c) allowing said depositedsol to create a gel, wherein said gel comprises a porous solid and apore fluid; and d) forming a dry, nanoporous dielectric by removing saidpore fluid without substantially collapsing said porous solid.
 7. Themethod of claim 6 wherein:the molar ratio of said first solventmolecules to the metal atoms in said reactant is no greater than 12:1.8. The method of claim 6, wherein:the molar ratio of said first solventmolecules to the metal atoms in said reactant is between 1:2 and 12:1.9. The method of claim 6, wherein:the molar ratio of said first solventmolecules to the metal atoms in said reactant is between 2.5:1 and 12:1.10. The method of claim 6, wherein:said nanoporous dielectric has aporosity greater than 60% and an average pore diameter less than 100 nm.11. The method of claim 6, wherein:said forming step is performed in adrying atmosphere, wherein the pressure of said drying atmosphere duringsaid forming step is less than the critical pressure of said pore fluid.12. The method of claim 6, wherein:the temperature of said substrateduring said forming step is above the freezing temperature of said porefluid.
 13. The method of claim 6, wherein:said forming step is performedin a drying atmosphere, wherein the pressure of said drying atmosphereduring said forming step is less than the critical pressure of said porefluid, the temperature of said substrate during said forming step isabove the freezing temperature of said pore fluid, and wherein, saidmethod does not comprise the step of adding a surface modification agentbefore said forming step.
 14. The method of claim 6, wherein:saidforming step is performed in a drying atmosphere, wherein the pressureof said drying atmosphere during said forming step is less than thecritical pressure of said pore fluid, the temperature of said substrateduring said forming step is above the freezing temperature of said porefluid, and said nanoporous dielectric has a porosity greater than 60%and an average pore diameter less than 100 nm; wherein, said method doesnot comprise the step of adding a surface modification agent before saidforming step.
 15. The method of claim 6, further compromising the stepof:aging said gel before said forming step.
 16. The method of claim 15,wherein:at least part of said aging step is performed in a substantiallyclosed container.
 17. The method of claim 15, wherein:the temperature ofsaid gel during said aging is greater than 30 degrees C.
 18. The methodof claim 15, wherein:the temperature of said gel during said aging isgreater than 80 degrees C.
 19. The method of claim 15, wherein:thetemperature of said gel during said aging is greater than 130 degrees C.20. The method of claim 6, wherein:said porous solid has less than 2%permanent volume reduction during said pore fluid removal.
 21. Themethod of claim 6, wherein:said porous solid remains substantiallyuncollapsed after said pore fluid removal.
 22. The method of claim 6,wherein:said porous solid has less than 5% volume reduction (includingtemporary volume reduction) during said pore fluid removal.
 23. Themethod of claim 6, wherein:said porous solid has less than 1% volumereduction (including temporary volume reduction) during said pore fluidremoval.
 24. The method of claim 6, wherein:said allowing step isperformed in a gelation atmosphere, wherein the concentration of a vaporof said first solvent in said gelation atmosphere is passivelycontrolled.
 25. The method of claim 6, wherein:said allowing step isperformed in a gelation atmosphere, wherein the concentration of a vaporof said first solvent in said gelation atmosphere is substantiallyuncontrolled.
 26. The method of claim 6, wherein:said reactant is ametal alkoxide selected from the group consisting oftetraethylorthosilicate, tetramethylorthosilicate,methyltriethoxysilane, 1,2-Bis(trimethoxysilyl)ethane and combinationsthereof.
 27. The method of claim 6, wherein:said reactant istetraethylorthosilicate.
 28. The method of claim 6, wherein:said dry,nanoporous dielectric has a porosity greater than 60%.
 29. The method ofclaim 6, wherein:said dry, nanoporous dielectric has a porosity between60% and 90%.
 30. The method of claim 6, wherein:said dry, nanoporousdielectric has a porosity greater than 80%.
 31. The method of claim 6,wherein:said dry, nanoporous dielectric has a porosity greater than 30%.32. The method of claim 6, wherein:said dry, nanoporous dielectric has aporosity greater than 45%.
 33. The method of claim 6, wherein:said dry,nanoporous dielectric has a porosity between 15% and 40%.
 34. The methodof claim 6, wherein:said precursor sol comprises a gelation catalyst.35. The method of claim 6, further comprising the step of:adding agelation catalyst after said deposition.
 36. The method of claim 6,further comprising the step of:replacing at least part of said porefluid with a liquid before said removing pore fluid step.
 37. The methodof claim 36, wherein:said liquid comprises acetone.
 38. The method ofclaim 6, further comprising the step of:annealing said dry, nanoporousdielectric.
 39. A method for forming a thermal insulator on asemiconductor substrate, the method comprising the steps of:a) providinga semiconductor substrate; b) depositing an aerogel precursor sol uponsaid substrate; wherein said aerogel precursor sol comprisesametal-based aerogel precursor reactant, wherein said reactant is acompound containing metal atoms, and a first solvent comprisingglycerol; wherein,the molar ratio of said first solvent molecules to themetal atoms in said reactant is at least 1 first solvent molecule per 16metal atoms; c) allowing said deposited sol to create a gel, whereinsaid gel comprises a porous solid and a pore fluid; and d) forming adry, thermal insulator by removing said pore fluid.